Attenuators of various different types are used throughout communication equipment for adjusting the power level of carrier signals. Because optical amplifiers are becoming ubiquitous in fiber optic systems for increasing the power level of optical carrier signals, variable optical attenuators are becoming increasingly important for adjusting the power level of optical communication signals. Such variable optical attenuators are particularly important for optical cross-connect fiber optic switches because optical signals may arrive at the optical switch from different places and therefore may have differing signal strengths.
Patent Cooperation Treaty (“PCT”) international patent application WO 00/20899 published 13, Apr. 2000, entitled “Flexible, Modular, Compact Fiber Optic Switch,” (“the '899 PCT patent application”) describes an optical cross-connect for switching quasi-collimated, free-space light beams. The '899 PCT patent application is hereby incorporated by reference as though fully set forth here. FIG. 1 illustrates one embodiment of an N×N optical switching module, indicated by the general reference character 100 and described in the '899 PCT patent application, that may be included in the fiber optic switch. The N×N optical switching module 100 includes two arrays 118a and 118b of movable mirror surfaces 116a and 116b of light beam deflectors that face each other.
As described in the '899 PCT patent application, each mirror surface 116 of the light beam deflectors is preferably provided by a two-dimensional (“2D”) torsional scanner of a type similar to those described in U.S. Pat. No. 5,629,790 (“the '790 patent”), and in PCT international patent application WO 00/13210 published 9 Mar. 2000 entitled “Micromachined Members Coupled For Relative Rotation By Torsional Flexure Hinges” (“the '210 PCT patent application”). Each 2D torsional scanner includes the mirror surface 116 which is coupled to and supported from an encircling frame by a first pair of hinges. The first pair of hinges permit the mirror surface 116 to rotate about a first axis with respect to the encircling frame. In turn, the encircling frame of the torsional scanner is itself coupled to and supported from an outer reference frame by a second pair of hinges. The second pair of hinges permit the encircling frame to rotate with respect to the outer reference frame about a second axis that is not oriented parallel to the first axis.
Each optical fiber 106 connected to the optical switching module 100 in either of its two (2) sides 102a and 102b can direct a beam of light 108 through a lens 112 to a unique entrance mirror surface 116a or 116b that is rotatable about the two non-parallel axes. Correspondingly, each optical fiber 106 may also receive a beam of light 108 that reflects from a unique exit mirror surface 116a or 116b. Each entrance mirror surface 116 in one array 118a or 118b can be rotated to point the beam of light 108 impinging thereon to any of the mirror surfaces 116 in the other array 118b or 118a. To couple a beam of light 108 through the free-space between a pair of optical fibers 106, i.e. one optical fiber 106 respectively from each of the sides 102a and 102b, the beam of light 108 from one of the optical fibers 106 in the side 102a or 102b impinges upon an entrance mirror surface 116 in the array 118a or 118b, reflects off the entrance mirror surface 116a or 116b to impinge upon a second exit mirror surface 116b or 116a in the array 118b or 118a, and to then reflect therefrom into one of the optical fibers 106 in the side 102b or 102a. 
The loss of optical power in the beam of light 108 coupled between pairs of optical fibers 106 connected to the optical switching module 100 depends critically on the respective orientations of the pair of mirror surfaces 116a and 116b in the light beam deflectors. Other elements surrounding the optical switching module 100 may also increase the amount of optical power loss.
To precisely align the orientations of the mirror surfaces 116a and 116b of the light beam deflectors, the fiber optic switch includes a dual axis servo controller 122 for each pair of mirror surfaces 116a and 116b that couple a beam of light 108 between a pair of optical fibers 106. FIG. 2 illustrates one channel of the dual axis servo controller 122.
As part of the dual axis servo controller 122, each optical fiber 106 of the fiber optic cross-connect switch includes a directional coupler 124 for tapping off a fixed amount of the optical signal power, e.g. a 20 dB optical coupler. The optical signal extracted by each directional coupler 124 impinges upon a photo-detector 126. Each photo-detector 126 receives and measures the optical power present in a fixed fraction of beam of light 108 propagating through the optical switching module 100 along the optical fibers 106 regardless of whether the optical fiber 106 is an incoming or an outgoing optical fiber 106. Precisely aligning the orientations of a pair of the mirror surfaces 116a and 116b of the light beam deflectors causes as much as possible of the beam of light 108 emitted from the incoming optical fiber 106 to propagate along the outgoing optical fiber 106.
Between the directional coupler 124 on the incoming optical fiber 106 and the optical switching module 100, and also past the directional coupler 124 on the outgoing optical fiber 106, there may exist other optical elements, such as additional couplers, switches, optical amplifiers, connectors and cables, all of which contribute to loss (or gain) of optical signal power through the fiber optic switch. FIG. 2 depicts the presence of these other optical elements respectively with the loss elements 128a and 128b. Furthermore, in addition to the loss elements 128a and 128b there may also exist loss elements, not illustrated in FIG. 2, which precede the directional coupler 124 on the incoming optical fiber 106, and are located between the optical switching module 100 and the directional coupler 124 on the outgoing optical fiber 106.
The input and output power levels measured by the photo-detectors 126 are supplied as input signals to the dual axis servo controller 122. The dual axis servo controller 122 uses these signals for properly orienting the pair of mirror surfaces 116a and 116b. The dual axis servo controller 122 may implement various different servo control algorithms for controlling orientation of the mirror surfaces 116a and 116b. 
As stated above, optical signals may arrive at the optical switching module 100 via the optical fibers 106 from different places and therefore may have differing signal strengths. Furthermore, differing wavelength optical signals may arrive at the optical switching module 100 on differing optical fibers 106. Such multiple beams of light having differing wavelengths, after passing through the optical switching module 100, may be multiplexed onto a single outgoing optical fiber. If optical signals having differing signal strengths are multiplexed together without controlling their respective strengths, wavelengths having different strength may increase differently during subsequent optical amplification. For this and other reasons it highly desirable that all wavelengths being multiplexed into a single optical fiber have approximately the same power.
In principle, such matching of the respective strengths of the optical signal carried by a set of outgoing optical fibers 106 can be accomplished by parsing each beam of light 108 through an attenuator located between an incoming optical fiber 106 and the outgoing optical fiber 106. However, because another fiber optic switch located elsewhere in the telecommunication system can, at any time, switch an incoming optical fiber 106 to a different optical signal source having a different signal strength, an attenuator included in the optical switching module 100 must be easily and quickly adjusted for appropriately attenuating optical signals of various strengths.